Non-synchronized ADPCM with discontinuous transmission

ABSTRACT

A method for coordinating an encoder and a decoder in a wireless communication system employing discontinuous transmission, the method comprising performing a syncless reset command on the encoder and the decoder, in order to bring the encoder and decoder into stable and compatible states. The syncless reset command can be implemented by setting the variables of the encoder and decoder into predetermined values, or by performing a normal reset command followed by encoding or decoding a predetermined number of predetermined sample values.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 371 as a national stagefiling of application no: PCT/IL2010/000966 filed on Nov. 18, 2010 andpublished as WO 2012/066525 the disclosure of which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to wireless communication in general, andto a method and system for implementing non-synchronized ADPCM withdiscontinuous transmission (DTX), in particular.

BACKGROUND

Consumer products such as communication devices and in particularwireless telephones have long become standard commodity.

In such systems, the transmitting side encodes the communicated data,while the receiving side decodes it. It will be appreciated that thetransmitting and receiving sides may alternate according to the speakingside. Many of these devices use Adaptive Differential Pulse CodeModulation (ADPCM) codecs, which are waveform codecs, in which theencoder instead of quantizing the speech signal directly, quantizes thedifference between the speech signal and a prediction that has been madeof the speech signal. If the prediction is accurate then the differencebetween the real and predicted speech samples is of lower variance thanthe real speech samples, and is accurately quantized with fewer bitsthan would be needed to quantize the original speech samples. At thedecoder side, the quantized difference signal is added to the predictedsignal to give the reconstructed speech signal.

In order to improve and make the usage of the transmitting side andreceiving side of a communication system more efficient, discontinuoustransmission (DTX) may be used. DTX is a method for reducingtransmission and thus optimizing the overall efficiency of wirelessvoice communications systems, by momentarily powering-down or muting anyof the sides, when no voice activity is detected.

In a typical two-way conversation, each individual speaks on averageslightly less than half of the time. If the transmitter signal isswitched on only during periods of voice input, the duty cycle of thesystem can be cut to less than 50 percent on average. This conservesbattery power and radiation, eases the workload of the components in thetransmitter amplifiers, and reduces interference.

As explained above, in ADPCM the encoding and decoding are notstateless, but rather recently communicated data is used during encodingand decoding current data. Therefore, after the two sides do notcommunicate for a while, getting back into communication introduces asynchronization problem.

Some prior art solutions include halting all activity of the encoder anddecoder so that they remain at the same state. The drawback of thissolution is that it is hard to ensure that once communication resumes,encoding and decoding start at the same sample, since even a very smalldeviation causes severe noises.

Other solutions include encoding and decoding predetermined orartificial data such as null data. This solution is more stable, butstill requires the encoder and decoder to fully operate even when thereis no data is transmission.

Yet another solution relates to introducing comfort noise generated by acomfort noise generator (CNG) to the encoder and the decoder, and havethe encoder and decoder operate on the comfort noise.

However, none of these solutions provides satisfactory synchronizationas well as processing power savings by avoiding unnecessary encoding anddecoding.

There is thus a need in the art for a method and system forsynchronizing the two sides in ADPCM with TDX communication systems.

SUMMARY

In a communication system employing discontinuous transmission betweenan encoder unit and a decoder unit, coordinating the encoder unit andthe decoder unit and the decoder after a silence period.

A first aspect of the disclosure relates to a method for coordinating anADPCM-based encoder comprised in a transmitting unit of a communicationsystem and an ADPCM-based decoder comprised in a receiving unit of thecommunication system after a silence period, wherein the communicationsystem employs discontinuous transmission, the method comprising: theencoder performing a first syncless reset operation consequent toreceiving audio data; the encoder operating on the audio data andsetting one or more encoded values in a buffer of samples; thetransmitting unit transmitting the content of the buffer of samples; thereceiving unit receiving the content of the buffer of samples; thedecoder performing a second syncless reset operation; and the decoderdecoding the received content. Within the method, the first synclessreset operation or the second syncless reset operation optionallycomprise setting internal variables of the encoder or the decoder topredetermined values. Within the method, the first syncless resetoperation or the second syncless reset operation optionally comprises:the encoder or the decoder performing a normal reset operation; and theencoder or the decoder encoding or decoding a predetermined number ofpredetermined sample values. Within the method, the predetermined numberis optionally equal to or larger than about 10. Within the method, thepredetermined sample value is optionally ‘0’ for the encoder andoptionally ‘F’ for the decoder. The method can further comprise thedecoder outputting the decoded received content. The method can furthercomprise the encoder setting one or more hard-coded values in the bufferof samples prior to performing the first syncless reset operation.

Another aspect of the disclosure relates to a communication systemcomprising a transmitting unit and a receiving unit, the communicationsystem employing DTX between the transmitting side and the receivingside, the transmitting unit comprising: a voice activity detector (VAD)for identifying whether an input signal is voiced or silent andtransmitting a VAD signal indicating whether the input signal is voicedor silent; an ADPCM-based encoder, comprising a syncless reset componentfor executing a first syncless reset command when the VAD signalindicates that the input signal changed from silent to voiced; thereceiving unit comprising: an ADPCM-based decoder, comprising a synclessreset component for executing a second syncless reset command when theVAD signal indicates that the input signal changed from silent tovoiced. Within the communication the encoder or decoder are optionallyadapted to set internal variables of the encoder or the decoder topredetermined values for executing the first syncless reset command orthe second syncless reset command. Within the communication system, forexecuting the first syncless reset command or the second syncless resetcommand the encoder or decoder are optionally adapted to: performing anormal reset command; and encoding or decoding a predetermined number ofpredetermined sample values. Within the communication system, thepredetermined number is optionally equal to or larger than about 10.Within the communication system, the predetermined sample value isoptionally ‘0’ for the encoder and optionally ‘F’ for the decoder.Within the communication system, the receiving unit can furthercomprise: a comfort noise generator for generating comfort noise whenthe input signal is silenced; and a switch for activating theADPCM-based decoder when the input signal is voiced, and the comfortnoise generator when the input signal is silent. Within thecommunication system, the encoder is optionally adapted to set one ormore hard-coded value in the buffer of samples prior to performing thefirst syncless reset command.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with thedrawings in which corresponding or like numerals or characters indicatecorresponding or like components. Unless indicated otherwise, thedrawings provide exemplary embodiments or aspects of the disclosure anddo not limit the scope of the disclosure. In the drawings:

FIG. 1 is a general scheme of a communication system comprising a fixedpart and a portable part;

FIG. 2 is a schematic illustration of the synchronization problembetween the fixed part and the portable part when DTX communication isused;

FIG. 3 is a general scheme of a communication system comprising atransmitting side and a receiving side, with a syncless reset, inaccordance with the disclosure;

FIG. 4A is a flowchart of the operation of the transmitting side whensyncless reset is used, in accordance with the disclosure;

FIG. 4B is a flowchart of the operation of the receiving side whensyncless reset is used; and

FIG. 5 is a graph comparing the performance of the syncless reset withother methods for a step signal; and

FIGS. 6A-6C show graphs comparing the performance of the syncless resetwith other methods for a speech signal, in accordance with thedisclosure.

DETAILED DESCRIPTION

A method and communication device which provide unsynchronized reset(syncless reset) commands in order to coordinate the transmitter andreceiver in a device having a transmitting side and a receiving sidewhich communicate in Adaptive Differential Pulse Code Modulation (ADPCM)with TDX protocol. When DTX methods are used, transmission is stoppedwhen no audio is input, the encoder on the transmitting side can keepupdating its state when silent samples and then voiced samples arereceived, while the decoder on the receiving side can not do the same,and synchronization is lost.

The syncless reset command causes the encoder at the transmitting sideand the decoder at the receiving side to achieve a stable point after aperiod of silence, by bringing the encoder and the decode to stablecompatible states, such that the output signal is the same as theoriginal one. The need arises since after a period of silence, theencoder and decoder may be at different and therefore non-compatibleinternal states, which cause the decoder to decode the encoded signalinto a signal significantly different than the original signal, whichmay cause noises and “clicks”, since the system diverges.

When a normal reset command is sent to a vocoder, i.e., an encoder ordecoder, the vocoder has to process 64 samples of silence in order toreach a stable point.

In order to imitate that effect, the syncless reset can be implementedin a number of options. A first implementation comprises setting theinternal variables of the vocoder into the same values, as would beobtained after processing 64 samples of silence consequent to a normalreset command.

Another option relates to each vocoder upon receiving a syncless resetcommand, artificially processing a predetermined number of artificialsilent samples, whether these samples have been received or not. It willbe appreciated that processing about 64 samples will reach the desiredeffect, similarly to the normal reset command. However, processing alower number, e.g., 10 samples consequent to the normal reset commandwill practically provide a stable result as well. It will be appreciatedthat once a syncless reset command has been received, processing thepredetermined number of samples can be performed without delay since thevocoder is not processing real incrementally-arriving data but ratherpredetermined data.

Referring now to FIG. 1, showing a schematic illustration of a prior artcommunication device, comprising a transmitting unit 100 and a receivingunit 102. If the communication device comprises a fixed unit and aportable unit, then either one of them is transmitting at times andreceiving at times.

Transmitting unit 100 comprises an ADPCM-based encoder 116, andreceiving unit 102 comprises an ADPCM-based decoder 132. Encoder 116,once in stable state, encodes a ‘0’ input sample into an ‘F’ sample. Onthe receiving side, decoder 132, once in stable state, decodes areceived ‘F’ sample into an output ‘0’ sample.

Transmitting unit 100 receives input signal 103, which is processed byvoice activity detection (VAD) unit 104 which detects whether the inputsignal is voiced or silent, and issues corresponding VAD 0/1 signal 108,which is for example 1 if audio is detected, and 0 otherwise.

The speech is processed by ADPCM encoder 116, and the output istransferred to switch 120 which also receives VAD 0/1 signal 108. If VAD0/1 signal 108 is indicates voiced input, the output of ADPCM encoder116, is transmitted to receiving unit 102. When DTX is used, if VAD 0/1signal 108 indicates silent input, transmission can be eliminated.

On receiving unit 102, if VAD 0/1 signal 108 indicates voiced input, thesignal is transferred to switch 128. If VAD 0/1 signal 108 indicatesvoiced input, the received signal is passed to ADPCM decoder 132 whichdecodes the signal. If VAD 0/1 signal 108 indicates silent input 0,comfort noise is generated by comfort noise generator (CNG) 136.

In accordance with the value of VAD 0/1 signal 108, switch 140 selectsfor output either the decoded audio or the generated comfort noise.

Referring now to FIG. 2, showing a schematic illustration of thesynchronization problem between transmitting unit 100 and receiving unit102 when using DTX.

Data transfer between transmitting unit 100 and receiving unit 102 isgenerally in bulks of a predetermined number of samples, e.g., 80samples. ADPCM encoder 116 processes the incoming signal, writes a valuecorresponding to each sample in the bulk, and when the bulk is fulltransmits it to the receiving side.

As long as no voice is detected by VAD unit 104, ADPCM encoder 116places a predetermined hard-coded value, for example ‘F’ (0x1111) foreach sample, such as locations 208 . . . 212 of bulk 200. Once speech isdetected and ADPCM-based encoder 116 starts receiving audio at timeindicated by arrow 204, encoder 116 continues to fill bulk 200 withactual encoded data X within locations 216 . . . 220. It will beappreciated that X does not relate to a particular value but ratherdenotes changing values stored within locations 216 . . . 220. It willbe appreciated that one or more X values can be equal to ‘F’. Inparticular, even the first sample after the silence can have a value of‘F’, which makes it undifferentiable from the silent ‘F’s preceding it.

Once bulk 200 is full, it is transmitted to receiving unit 102, which isthus notified that communication is resumed. Receiving unit 102 is,however, unaware of when transmitting unit 100 started receiving audio,and can therefore not determine when to start processing the receivedbulk, since each ‘F’ can be a predetermined value or an actual value.Thus, transmitting unit 100 and receiving unit 102 are not synchronizedwhich may result in non-matching states and output signal that issignificantly different from the input signal.

Referring now to FIG. 3, showing a schematic illustration of a solutionto the synchronization problem. The system comprises transmitting unit100 and receiving unit 102 as in FIG. 1. In addition, as thetransmission mode changes from “Off” to “On”, each of encoder 116 anddecoder 132 execute a syncless reset command by syncless reset component300 which performs operations as detailed below. It will be appreciatedthat component 300 can be implemented using software, and/or hardwareand/or firmware or any other known technology.

In some embodiments, encoder 116 and decoder 132 always run on realdata: the encoder encodes only real signal, while the decoder runs onreal data when transmission is on, and on arbitrary data generated bycomfort noise generator 136 when transmission is off. Thus, no nullsamples are input into encoder 116 or decoder 136. In alternativeembodiments, encoder 116 and decoder 136 can avoid running whentransmission is off. In any case, it does not matter whether the encoderand decoder operate or not. Further, if the encoder and decoder do work,it does not matter what data they work on, and there is no requirementfor running on particular data.

The syncless reset command is aimed at bringing the encoder and thedecoder to stable and compatible states, whatever data they were runningon before, at which states their variables are not changed if the inputis ‘0’ for the encoder or ‘F’ for the decoder, no matter how long the‘F’ or ‘0’ sequence is.

Referring now to FIGS. 4A and 4B, showing schematic flowcharts ofmethods for coordinating the transmitting and receiving sides after asilent period.

FIG. 4A shows a schematic flowchart of the operation of the transmittingside. On step 400 which is continuously occurring during the silentperiod, the encoder fills a buffer or a bulk with values indicatingsilence, such as ‘F’s.

On step 404 the encoder starts receiving an input signal, for exampleonce VAD detected voiced input. On step 408 the encoder performs asyncless reset command. The syncless reset command places the encoder inthe same stable point as achieved after a long period of silence input.

The syncless reset command can be performed in a number of embodiments.In one embodiment, comprising step 412, the encoder sets its variablesinto a set of predetermined values, which are substantially the samevalues as those obtained after the encoder has processed a number of ‘0’input samples. The variable names, their values in normal reset command,and the values in the syncless reset command are detailed on table 1.Table 2, however, lists only the variables whose assigned values differbetween a normal reset command and a syncless reset command. The othervalues are the same as in the normal reset command, as provided by thecurrent or future ITU G.726 standard for ADPCM for the normal resetoperation.

TABLE 1 Value in normal Value in syncless reset Variable name resetoperation operation inp_buf 0x004417b0 0x004417b0 out_buf 0x004415800x00441580 smpno 0x00000100 0x00000100 law 0x0012fe40 “2” 0x0012fe40 “2”rate 0x0004 0x0004 r 0x0000 0x0000 state 0x0012ff18 0x0012ff18 sr00x0020 0x0020 sr1 0x0020 0x0020 a1r 0x0000 0x0000 a2r 0x0000 0x0000 b1r0x0000 0x0000 b2r 0x0000 0x0000 b3r 0x0000 0x0000 b4r 0x0000 0x0000 b5r0x0000 0x0000 b6r 0x0000 0x0000 dq5 0x0020 0x0420 dq4 0x0020 0x0420 dq30x0020 0x0420 dq2 0x0020 0x0420 dq1 0x0020 0x0420 dq0 0x0420 0x0420 dmsp0x0000 0x0000 dmlp 0x0000 0x0000 apr 0x0020 0x01f1 yup 0x0220 0x0220 tdr0x0000 0x0000 pk0 0x0000 0x0000 pk1 0x0000 0x0000 ylp 0x000088000x00008800 u4 0x0001 0x0000 b1 0x0000 0x0000 a2t 0x0000 0x0000 dms0x0000 0x0000 dqs 0x0001 0x0001 tdp 0x0000 0x0000 dx 0xcccc 0xcccc u30x0001 0x0000 wb6 0x0000 0x0000 u2 0x0001 0x0000 pk2 0x0000 0x0000 wb50x0000 0x0000 sez 0x0000 0x0000 dsx 0xcccc 0xcccc wi 0x0ff4 0x0ff4 u10x0001 0x0000 a2 0x0000 0x0000 b2p 0x0000 0x0000 b6p 0x0000 0x0000 wb40x0000 0x0000 td 0x0000 0x0000 a1 0x0000 0x0000 a2p 0x0000 0x0000 a1t0x0000 0x0000 wb3 0x0000 0x0000 ap 0x0000 0x01f1 sr2 0x0020 0x0020 wb20x0000 0x0000 yut 0x0203 0x0203 j 0x00000001 0x00000041 i 0x000f 0x000fy 0x0220 0x0220 tr 0x0000 0x0000 dlnx 0xcccc 0xcccc wb1 0x0000 0x0000 se0x0000 0x0000 dqln 0x0800 0x0800 b5p 0x0000 0x0000 dml 0x0000 0x0000 dql0x0888 0x0888 sd 0xcccc 0xcccc slx 0xcccc 0xcccc yl 0x000088000x00008800 fi 0x0000 0x0000 dq 0x8000 0x8000 a1p 0x0000 0x0000 b1p0x0000 0x0000 app 0x0020 0x01f1 al 0x0000 0x0040 wa2 0x0000 0x0000 sigpk0x0001 0x0001 sr 0x0000 0x0000 wa1 0x0000 0x0000 b6 0x0000 0x0000 b4p0x0000 0x0000 sp 0xcccc 0xcccc b5 0x0000 0x0000 so 0x0000 0x0000 ax0x0001 0x0001 b4 0x0000 0x0000 dq6 0x0020 0x0420 u6 0x0001 0x0000 b30x0000 0x0000 dlx 0xcccc 0xcccc yu 0x0220 0x0220 u5 0x0001 0x0000 b20x0000 0x0000 b3p 0x0000 0x0000

TABLE 2 Value in normal Value in syncless reset Variable name resetoperation operation u4 0x0001 0x0000 dqs 0x0001 0x0001 u3 0x0001 0x0000u2 0x0001 0x0000 u1 0x0001 0x0000 ap 0x0000 0x01f1 app 0x0020 0x01f1 al0x0000 0x0040 dq6 0x0020 0x0420 u6 0x0001 0x0000 u5 0x0001 0x0000

It will be appreciated that for some variables setting valuessubstantially similar to the values shown for the syncless resetoperation in Table 1 can also provide stable results, and is alsocovered by the present disclosure. For example, the “al” variable is acounter and can be set to any value.

In another embodiment, normal reset is performed by the encoder on step416, followed by processing a predetermined number of ‘0’ input sampleson step 420. The predetermined number can be between about 10 and about64, after which the encoder assumes a substantially stable state notaffected by subsequent ‘0’ samples. It will be appreciated that sincethe processing is not performed upon real data but upon predeterminedvalues, it can be completed in a fraction of the time it would take toprocess the same number of real input sample, and would thus not causeany additional delay beyond the delay of a system without Synclessreset.

Once the encoder is at a stable state, on step 424 it continues toprocess the actual audio data received and to fill the buffer, and onstep 428 when the bulk is full, the bulk is transmitted to the receivingside. The bulk may thus comprise a number of ‘F’ samples generated dueto no audio input, followed by a number of real data samples.

FIG. 4B shows a schematic flowchart of the operation of the receivingside. On step 440 the decoder receives a first bulk after a silenceperiod in which no transmissions were received.

On step 444 the decoder performs syncless reset, in order to achieve astable state, so that regardless of the number of ‘F’ samples in thereceived bulk, the encoder will be in the same stable point as achievedafter processing a long period of silent input.

The syncless reset, similarly to the syncless reset performed by thetransmitting side on step 408, can be performed in a number ofembodiments. In one embodiment, comprising step 448, the decoder setsits variables into a set of predetermined values, which aresubstantially the same values as those obtained after the decoder hasprocessed a number of ‘F’ input samples. The values are substantiallyidentical to the values of the variables in the encoder as set on step412.

Alternatively, in another embodiment, normal reset is performed by thedecoder on step 452, followed by processing a predetermined number of‘F’ input samples on step 456. The predetermined number can be betweenabout 10 and about 64, after which the decoder assumes a substantiallystable state not affected by subsequent ‘F’ samples. It will beappreciated that since the processing is not performed upon real databut upon predetermined values, it can be completed in a fraction of thetime it would take to process the same number of real input samples, andwould thus not cause delay.

Once the decoder is in a stable state, on step 460 the decoder operateson the received bulk, and remains stable, and thus in accordance withthe state the encoder was at when encoding the signal, no matter howmany ‘F’ samples are at the beginning of the bulk, resulting from thesilent period.

The syncless reset operation solves matching and coordination issuesbetween the encoder and decoder, since the encoder and decoder are atthe same state when encoding and decoding the real audio data, no matterhow many predetermined samples are at the beginning of the transmittedbulk.

If a normal reset command was used instead of the described synclessreset, a noise such as a loud “click” would sound on the receiving sidedue to the lack of synchronization and the difference in states betweenthe encoder and decoder. The disclosed embodiments also provide forfaster stabilization time after the reset.

Referring now to FIG. 5, showing graphs demonstrating the convergence ofthe output signal to the original signal for a step input signal, whenusing the normal reset method and when using the syncless reset method.The original signal is denoted 508, and the signal following a synclessreset as described above is denoted 512. In comparison, the signal aftera synchronized normal reset is denoted 516, and the signal after anon-synchronized normal reset, i.e. when there is a mismatch of at leastone sample between the encoder and the decoder is denoted 520. It isseen that the syncless reset provides for faster stabilization than thesynchronized normal reset. It is also seen that the non-synchronizednormal reset, as shown by line 520 provides even worse convergence rateand higher levels of noise than synchronized normal reset. Thenon-synchronized normal reset first causes a spike 524, and thenconverges to a wrong signal, which may happen even because of a singlesample mismatch. Signal 512, showing the convergence of the outputsignal to the original signal following a syncless reset command doesnot change, regardless of the length of the sample mismatch.

Referring now to FIGS. 6A-6C, showing the errors resulting in differentalgorithms for a speech signal. FIG. 6A shows the error between an inputspeech signal and the signal output when using the normal reset commandwhen the encoder and decoder are fully synchronized.

FIG. 6B shows the error between the input speech signal and the outputsignal when using the normal reset command when the encoder and decoderare not synchronized, i.e. the encoder and decoder operate on sequencesdiffering in at least one sample, and FIG. 6C shows the error betweenthe input speech signal and the output signal when using the synclessreset command. The graph shown in FIG. 6C will not change regardless ofthe length of the sample mismatch.

For speech signal, the syncless reset command provides convergence whichis about as fast and as good as the normal reset when synchronized, andmuch better performance, including lack of spikes, than the normal resetcommand when the encoder and decoder are not synchronized.

In addition to fast and good stabilization, the syncless reset alsoprovides for similar performance regardless of possible mismatch betweenthe encoder and decoder, i.e., no matter how many artificial orhard-coded ‘F’ samples are at the beginning of the transmitted bulk, theoutput stabilizes just the same.

The disclosed methods provide for solving the problem of lack ofsynchronization between an encoder and a decoder in discontinuoustransmission. The problem is caused since the encoder first fills a bulkto be transmitted with artificial or hard-coded data, and once audioinput is received continues to fill the bulk with real data; the decoderhowever, starts operating once it receives the bulk, but can not knowwhere exactly in the bulk the hard-coded data ends and the real databegins. Thus, the encoder and decoder may not be in the same state whenthe real data starts, resulting in a signal output by the decoder whichmay be significantly different from the signal input into the encoder.

The disclosed methods provide for bringing the encoder and the decoderinto identical and stable states, which are the same states that areachieved after long periods of silence, so that no matter how manyhard-coded sample values are there at the beginning of the bulk, theoutput signal converges quickly to the input signal. The disclosedmethods provide high stability of the encoder and decoder, and even amismatch in the number of artificial values entered to the encoder andto the decoder do not cause the system to divert, unlike regular reset,in which an incompatible number of artificial values causes changes inthe internal parameters and therefore loss of synchronization. This isachieved without stopping the encoder or the decoder, and withoutfeeding the system with any required values.

It will be appreciated that the disclosed arrangement is not limited toany type of devices, and can be used also for any other environments,devices or audio devices such as wireless communication devices, IPcommunication devices, recorder, player, phone, baby monitor, securitydevice, media recorder or player, voice over IP devices, or the like. Itwill also be appreciated that the disclosed methods include setting theencoder and decoder into values close to the values shown in table 1above, which may also provide good estimation of a stable state.

It will be appreciated that the disclosure can be implemented usingsoftware, and/or hardware and/or firmware or any other known technology.The parameters can either be set directly to the values detailed above,or by using a normal reset command followed by processing 10-64 silencesamples, having for example values of ‘0’ for the encoder and ‘F’ forthe decoder.

While the disclosure has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particularsituation, material, step of component to the teachings withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosed subject matter not be limited to the particularembodiment disclosed as the best mode contemplated for carrying out thisinvention, but only by the claims that follow.

What is claimed is:
 1. In a communication system comprising atransmitting unit including an Adaptive Differential Pulse CodeModulation (ADPCM)-based encoder and a receiving unit including anADPCM-based decoder, the communication system employing discontinuoustransmission (DTX), a method for coordinating the ADPCM-based encoderand the ADPCM-based decoder after an end of a silence period duringwhich the transmitting unit did not transmit, the method comprising: theADPCM-based encoder performing a first syncless reset operation inresponse to a first reception of audio data after the end of the silenceperiod; the ADPCM-based encoder operating on the audio data and settingat least one encoded value in a buffer of samples; the transmitting unittransmitting the content of the buffer of samples; the receiving unitreceiving the content of the buffer of samples; the ADPCM-based decoderperforming a second syncless reset operation in response to thereception of the content of the buffer samples; and the decoder decodingthe received content; wherein said first syncless reset operationcomprises setting internal variables of the ADPCM-based encoder intosame values as would be obtained after performing, by the ADPCM-basedencoder, a normal reset operation and encoding a predetermined number ofADPCM-based encoder samples having a predetermined ADPCM-based encodersample value; wherein said second syncless reset operation comprisessetting internal variables of the ADPCM-based decoder respectively intosame values as would be obtained after performing, by the ADPCM-baseddecoder, a normal reset operation and decoding a predetermined number ofADPCM-based decoder samples having a predetermined ADPCM-based decodersample value.
 2. The method of claim 1, wherein the predetermined numberequals
 10. 3. The method of claim 1, wherein the predeterminedADPCM-based encoder sample value is ‘0’ and wherein the predeterminedADPCM-based encoder sample value is‘F’.
 4. The method of claim 1,further comprising the ADPCM-based decoder outputting the decodedreceived content.
 5. The method of claim 1, further comprising theADPCM-based encoder setting at least one hard-coded value in the bufferof samples prior to performing the first syncless reset operation.
 6. Acommunication system comprising a transmitting unit and a receivingunit, the communication system employing discontinuous transmission(DTX) between the transmitting side and the receiving side, thetransmitting unit comprising: a voice activity detector (VAD) foridentifying whether an input signal is voiced or silent and transmittinga VAD signal indicating whether the input signal is voiced or silent;wherein the transmitting unit is configured not to transmit during asilence period during which the input signal is silent; an AdaptiveDifferential Pulse Code Modulation (ADPCM)-based encoder, comprising afirst syncless reset component for executing a first syncless resetoperation when the VAD signal indicates that the input signal changedfrom silent to voiced; the receiving unit comprising: an ADPCM-baseddecoder, comprising a second syncless reset component for executing asecond syncless reset operation when first receiving a content of buffersamples from the transmitter after an end of the silence period; andwherein said first syncless reset component is configured to setinternal variables of the ADPCM-based encoder into same values as wouldbe obtained after performing a normal reset operation and encoding apredetermined number of ADPCM-based encoder samples having apredetermined ADPCM-based encoder sample value; wherein said secondsyncless reset component is configured to set internal variables of theADPCM-based decoder respectively into same values as would be obtainedafter performing, by the ADPCM-based decoder, a normal reset operationand decoding a predetermined number of ADPCM-based decoder sampleshaving a predetermined ADPCM-based decoder sample value.
 7. Thecommunication system of claim 6, wherein the predetermined number equals10.
 8. The communication system of claim 6, wherein the predeterminedADPCM-based encoder sample value is ‘0’ and wherein the predeterminedADPCM-based encoder sample value is‘F’.
 9. The communication system ofclaim 6, wherein the receiving unit further comprises: a comfort noisegenerator for generating comfort noise during the silence period; and aswitch for activating the ADPCM-based decoder after the silence periodends, and for activating the comfort noise generator during the silenceperiod.
 10. The communication system of claim 6, wherein the ADPCM-basedencoder is adapted to set at least one hard-coded value in the buffer ofsamples prior to performing the first syncless reset operation.
 11. Themethod of claim 1, wherein the predetermined number is equal to orlarger than
 64. 12. The communication system of claim 6, wherein thepredetermined number is equal to or larger than
 64. 13. The methodaccording to claim 1, comprising closing the ADPCM-based encoder and theADPCM-based decoder during the silence period.
 14. The method accordingto claim 1, comprising encoding by the ADPCM-based encoder only audiodata that is real.
 15. The method according to claim 1, wherein thesecond syncless reset operation is executed without feeding the receiverrequired values of content of the buffer of samples.
 16. The methodaccording to claim 1, wherein a duration of the first syncless resetoperation is a fraction of a duration required to decode, by theADPCM-based decoder, an amount of real samples that equals thepredetermined number of ADPCM-based decoder samples.
 17. Thecommunication system of claim 6, wherein the ADPCM-based encoder and theADPCM-based decoder are closed during the silence period.
 18. Thecommunication system of claim 6, wherein the ADPCM-based encoder isconfigured to encode only audio data that is real.
 19. The communicationsystem of claim 6, wherein the second syncless reset component isconfigured to perform the second syncless reset operation is executedwithout feeding the receiver required values of content of the buffer ofsamples.
 20. The communication system of claim 6, wherein a duration ofthe first syncless reset operation is a fraction of a duration requiredto decode, by the ADPCM-based decoder, an amount of real samples thatequals the predetermined number of ADPCM-based decoder samples.